A number of systems have been suggested for sensing the temperature within a refrigerated trailer and providing an output dependent upon the temperature within the trailer.
U.S. Pat. No. 4,024,495, for example, discloses a remote temperature-warning system for refrigerated vehicles that uses the existing electrical wiring of the vehicle. A visual temperature indicator is installed within the cab of the tractor to allow the driver to continuously monitor the temperature within the refrigerated compartment. An audio alarm is activated when the temperature measured within the refrigerated compartment exceeds a predetermined temperature.
U.S. Pat. No. 4,385,289 discloses a system for sensing the temperature within a refrigerated trailer and actuating an alarm when the temperature exceeds a preselected temperature. This system includes a control for switching between off, test and alarm modes of the system.
U.S. Pat. No. 4,463,348 discloses a microprocessor based monitor and display system for a refrigerator, which senses the actual time the refrigerator door is open, accumulates a count representative of the amount of door open time occurring in a plurality of successive time segments, calculates a rolling average of door open time, and can provide a visual display to indicate when the rolling time average is greater than, less than or within predetermined normal range.
U.S. Pat. No. 4,790,143 discloses a refrigeration monitoring system for truck trailers and containers, including programmable logic array, microcomputer or discrete gate logic systems connected with a plurality of sensors for temperatures from the refrigerated compartment for the air leaving and entering the evaporator, for compressor operating conditions and with a defrost signal. The monitor provides a warning if it detects marginal or inefficient operation and, after a preset time, a shutdown signal.
U.S. Pat. No. 4,798,055 discloses a portable, hand-operable refrigeration system analyzer. The disclosed apparatus includes a temperature probe, adapted for normal placement at each of a plurality of preselected points, in turn, around a refrigeration loop, connected by cable with a hand held means including a microprocessor for reading and storing probe temperature measurements, for storing standards and ranges of temperature data for properly operating refrigeration systems, and for comparison and diagnostic programs, and further including a key pad and alphanumeric display for entering data and operating the programs. The apparatus is provided with an output for feeding temperature and analysis data to external recording devices.
U.S. Pat. No. 4,882,564 discloses a system for remotely monitoring temperature including a temperature sensor, a transmitter unit located in the refrigerated transport container, and a display and alarm unit remote from the temperature sensing unit. The display and alarm unit includes a microprocessor, a RAM to store program instruction and data, and a RAM to store intermediate results and data. The display and alarm unit generates visual and aural alarms if the sensed temperature is outside of limits for a predetermined length of time.